The purpose of this study is the preparation of various kinds of cross-linked hemoglobins and analysis of their chemical and physiological properties. The cross-linking bifunctional reagents to be studied are dimethyl malonimidate, dimethyl adipimidate and diethyl suberimidate, homologues containing chains of increasing length. In addition to studies relating to the extent and manner of cross-linking of the polypeptide chains, these "polyhemoglobins" will be studied for potential use as plasma protein substitutes with particular reference to their life span in the plasma of rabbits. Since most red cells of out-dated human donor blood are now discarded, this study is aimed at exploring the usefulness of such cells in the preparation of a stable protein which might be useful as a plasma expander.